This invention relates to phosphor materials, such as those used in x-ray screens. In particular, it relates to a process for improving the properties of a rare-earth-activated yttrium, lanthanum, or gadolinium oxysulfide phosphor material that has been subjected to grinding, and to x-ray screens prepared using such a phosphor material.
It is well known that certain materials referred to as phosphors have the property of absorbing one type of electromagnetic radiation (exciting radiation) and emitting a second, usually lower energy, type of electromagnetic radiation. Thus calcium tungstate and other known phosphor materials have been used to convert x-ray image patterns into radiation which can be advantageously recorded on photographic film. The phosphor material is typicaly used in the form of a layer of phosphor particles, which comprise an x-ray screen. The phosphor particles forming the layer are typically imbedded in a binder matrix, and the layer may be coated on a support, such as a polymeric sheet.
In order to increase image resolution, decrease quantum mottle, and otherwise improve the final image when using an x-ray screen, it is desirable to use a phosphor material having uniform small particle size. In the preparation of phosphor materials it often happens that the phosphor as initially prepared comprises aggregations of individual phosphor particles. Various grinding methods known in the art can be used to break up such aggregations, and, if desired, to further reduce the size of individual phosphor particles; these include ball-milling, air impact pulverizing, etc.
However, certain phosphor materials are known to exhibit a decrease in luminescence after grinding. This is true for Lenard phosphors, such as zinc sulfide. U.S. Pat. No. 2,187,022, issued Jan. 16, 1940, describes a method of restoring the luminescence of such phosphor materials by heating the phosphor particles as they pass through a substantially inert gas, such as nitrogen.
U.S. Pat. Nos. 2,729,604 and 2,729,605, both issued Jan. 3, 1956, describe, respectively, bismuth- and antimony-activated ('604), and samarium-activated ('605), lanthanum oxychloride phosphors, and methods for their preparation. The described methods can involve multiple heating and pulverizing steps, including a calcining step at 800.degree. to 1100.degree. C.
U.S. Pat. No. 3,113,929, issued Dec. 10, 1963, describes a method for increasing the electroluminescence of electroluminescent phosphors, such as a phosphor comprising zinc sulfide (90%) -- zinc oxide (5%) -- magnesium oxide (5%), including heating such an electroluminescent phosphor to a temperature between about 700.degree. and 1000.degree. C in the presence of oxygen.
More recently, certain oxysulfide phosphor materials having useful cathodoluminescent and x-ray luminescent properties, and methods for their preparation, have been described in the art. For instance, U.S. Pat. Nos. 3,418,246 and 3,418,247, issued Dec. 24, 1968, and U.S. Pat. No. 3,705,858, issued Dec. 12, 1972, describe various rare-earth activated phosphor materials and methods for their preparation, including yttrium, lanthanum, and gadolinium oxysulfides. In the method of preparation described in U.S. Pat. No. 3,705,858, for instance, a precursor of the phosphor is precipitated from solution under carefully controlled conditions; the precipitate is then heated in a reducing atmosphere to form the phosphor, followed by annealing in an inert atmosphere, such as annealing in a covered crucible. While oxysulfide materials as described possess advantageous properties as phosphors, it is desirable to further improve their properties where possible.
For instance, in x-ray screens oxysulfide phosphor materials having a particle size in the range of about 1 micron to about 25 microns are useful. When oxysulfide phosphor materials are subjected to grinding to achieve the desired particle size, these phosphor materials exhibit a decrease in liminescence. Luminescence, as used herein, refers to electromagnetic radiation emissions from the phosphor material occurring concurrently with the period of exposure to exciting radiation. The adverse effects resulting from the grinding of oxysulfide phosphor materials are independent of the particular method of grinding used.
Also, it is indicated in the art that such oxysulfide phosphor materials are intolerant of heating in the presence of oxygen, such as in air. See, for instance, U.S. Pat. No. 3,864,273. In a study of the properties of selected europium-activated rare earth oxygen sulfur compounds reported in the J. Electrochem. Soc.: SOLID STATE SCIENCE, October, 1968, pp. 1060-1066, differential thermal analysis (DTA) data and thermogravimetric analysis (TGA) data indicated that oxidative decomposition of the yttrium, lanthanum, and gadolinium oxysulfides when heated in air begins to occur at temperatures between about 350.degree. and 595.degree. C, depending on the particular analysis used and the particular oxysulfide being tested. It was also reported in "Crystal Growth of Lanthanum Oxysulfide," by L. E. Sobon, presented at the A.C.C.G. Conference on Crystal Growth, Gaithersburg, Maryland, 1969, that lanthanum oxysulfide oxidizes when heated in air at 600.degree. C. It would thus appear undesirable to heat oxysulfide phosphor materials while exposed to oxygen-containing atmospheres, e.g., air, at temperatures above 600.degree. C.
Another property exhibited by oxysulfide phosphor materials is afterglow. Afterglow, as used herein, refers to the persistence of electromagnetic radiation emissions from the phosphor material after termination of the exciting radiation. In most situations, e.g., x-ray screens, it is desirable to minimize afterglow. Although the problem of afterglow can be aggravated by the presence of unwanted impurities, even relatively pure oxysulfide phosphors exhibit some afterglow.